The earliest uses of satellite communication systems employed frequency division multiple access (FDMA) techniques to allow multiple channels per carrier. These systems were optimized for medium to heavy communication links. However, as time passed, a need for light traffic links was recognized. The result was the development and implementation of demand-assigned single channel per carrier FDMA systems such as SPADE. The demand-assigned single channel per carrier links could employ the same repeater used for a multiple channel per carrier FDMA system inasmuch as the two techniques were and are compatible.
It has now been recognized that time division multiple access (hereinafter TDMA) enjoys certain advantages over frequency division multiple access and therefore at least several such communication systems are now beginning operation or are in the planning steps. As a result of this activity, applications calling for light or sporadic per station communication needs are becoming apparent.
To serve these particular stations with the standard TDMA hardware would be an inefficient and therefore expensive mismatch of facilities to the services required. This is for the reason that light traffic stations would require the high bit rate r-f equipment and precise time measuring equipment used in other stations to ensure compatibility between stations and maximum efficiency.
Assuming the presence of a baseline TDMA network, including several stations and a repeater in a quasi-stationary earth orbit, it would be possible to serve the sporadic and light traffic needs of certain stations with the demand-assigned single channel per carrier prior art techniques, using a frequency division multiple access mode. However, this would be incompatible with the desire to employ TDMA techniques. For one thing, separate repeater or relays would have to be set aside for the FDMA service. In addition, "hub" stations, which work into both the baseline and light nets would require duplicate r-f facilities for the TDMA and FDMA channels. Accordingly, retaining FDMA techniques for the sporadic and light traffic needs of certain stations is not an attractive alternative.
In accordance with the invention, these competing desires are satisfied by using a single repeater and using different bit rates for baseline and light traffic networks.
While a majority of conventional TDMA systems employ a common bit rate throughout, the prior art does evidence proposals to employ multiple bit rates in a single TDMA system. For example, see "A Time Division Multiple Access System for the Defense Satellite Communications System" by Husted and Walker, appearing in the report of the 1970 EASCON, pages 229-237. The arrangement disclosed in the aforementioned publication mentions a TDMA system including stations operating at two bit rates, with the bit rates having a 4:1 ratio. However, this system has a number of unique aspects which make it of more limited application. For example, the system is proposed for an arrangement which includes a spread spectrum communication system from which system timing is available. Normally, system timing is not available from an external source. As such, there is no frame reference burst in the disclosed arrangement. Furthermore, bit timing and carrier recovery is carried out at a common bit rate for all the system stations regardless of the bit rate at which they transmit and receive traffic information. Thus, the publication does not suggest a TDMA system in which two frame reference bursts are provided, each at a different bit rate, in that as disclosed, no frame reference burst is provided. Furthermore, the disclosed system employs a common bit rate for carrier and bit timing recovery and thus, the low bit rate stations must be capable of demodulating the high bit rate signal.
Accordingly, it is an object of the present invention to provide a TDMA system, served by a single repeater, which includes at least two networks, a baseline network comprised of stations optimized for medium to heavy traffic, and therefore, operating at relatively high bit rates, and at least a second network including stations which are optimized for light traffic, and therefore, operate at relatively lower bit rates. It is another object of the present invention to provide a communication system in which a first group of stations is capable of sending and receiving control, signalling and traffic information at a first bit rate, and a second group of stations which are not capable of sending or receiving at the first bit rate, but which instead transmit and receive control, signalling and traffic information at a second lower bit rate, in which both groups of stations communicate among themselves in a TDMA format through a single repeater. It is another object of the present invention to provide for the modifications of conventional TDMA networks to allow a second network including stations incapable of sending and receiving at the bit rate of the first network, to be "piggy-backed" onto the same repeater.